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PH-EP-2012-072

(Submitted on 20 Mar 2012)

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Abstract

A search for Bs -> mu+ mu- and B0 -> mu+ mu- decays is performed using 1.0
fb^-1 of pp collision data collected at \sqrt{s}=7 TeV with the LHCb experiment
at the Large Hadron Collider. For both decays the number of observed events is
consistent with expectation from background and Standard Model signal
predictions. Upper limits on the branching fractions are determined to be BR(Bs
-> mu+ mu-) < 4.5 (3.8) x 10^-9 and BR(B0 -> mu+ mu-) < 1.0 (0.81) x 10^-9 at
95% (90%) confidence level.

Figures and captions

Distribution of selected candidates (black points) in the (left) $ B^0_s \to\mu^+\mu^-$ and (right) $ B^0\to\mu^+\mu^-$
mass window for BDT$>$0.5, and expectations for, from the top, $ B^{0}_{(s)}\to\mu^+\mu^-$ SM signal (gray), combinatorial background (light gray), $ B^0_{(s)}\to h^+{h'}^{-}$ background (black), and cross-feed of the two modes (dark gray). The hatched area depicts the uncertainty on the sum of the expected contributions.

$\textrm{CL}_{\textrm{s}}$ as a function of the assumed $\cal B$ for (left) $ B^0_s \to\mu^+\mu^-$ and (right)
$ B^0\to\mu^+\mu^-$ decays.
The long dashed black curves are the medians of the expected $\textrm{CL}_{\textrm{s}}$ distributions for $ B^0_s \to\mu^+\mu^-$ , if background and SM signal were observed, and for $ B^0\to\mu^+\mu^-$ , if background only was observed.
The yellow areas cover, for each $\cal B$ , 34% of the expected $\textrm{CL}_{\textrm{s}}$ distribution on each side of its median.
The solid blue curves are the observed $\textrm{CL}_{\textrm{s}}$ . The upper limits at 90 % (95 %) CL are indicated by the dotted (solid) horizontal lines in red (dark gray) for the observation and in gray for the expectation.